1. Field of the Invention
The present invention relates to a battery charger and a battery charging method for charging a battery and particularly relates to a battery charger and a battery charging method suited for charging a battery, such as a nickel metal hydride battery, which emits high heat while being charged.
2. Discussion of the Prior Art
Presently, a chargeable battery which can be repeatedly used for the power supply of, for example, a power tool is used.
A nickel cadmium battery is popular as a battery for the power tool, and a battery charger for quickly charging the battery by applying high current thereto is used. Specifically, the battery is quickly charged in about 20 minutes and a power tool can be continuously used by switching a battery to that which has been charged.
The present inventor of the present invention studied  has investigated techniques for improving the performance of a power tool by using a nickel metal hydride battery as a battery therefor. Although the  nickel metal hydride battery can increase a  batteries have increased charge storage capacity as compared to a nickel cadmium battery, it generates high heat while being charged. If the temperature of the battery becomes high by  due to the generated heat, the electrodes and separators of the cells within the battery will deteriorate and battery life is  will be shortened. Due to this, it is  has been impossible to quickly charge the  a nickel metal hydride battery with  using high current as was previously done for the  nickel cadmium battery stated above  batteries.